There have been a number of research efforts to develop a gun-launch alternative for launching payloads into space. Potential advantages of gun launching can include relatively lower cost, shorter launch preparations, relatively rapid turnaround between successive launches, or relative insensitivity to weather conditions at the launch site (because the fastest, and therefore least vulnerable, portions of the launched projectile's flight occurs as it exits the barrel of the gun, so that ground-level weather has a lesser effect on the flight).
Previous efforts to develop a gun-launch system for launching payloads into space include the High Altitude Research Program (HARP) in the 1960's and the Super High Altitude Research Program (SHARP) in the late 1980's and early 1990's; both of those were research programs of the United States government. During the course of HARP a 185 lb. rocket body was launched at a muzzle velocity of 2,160 m/s (7,100 ft/s or about Mach 6), reaching an altitude of about 180 kilometers (591,000 ft). A 5 kg projectile was launched at a muzzle velocity of 3,000 m/s (6,700 mph or about Mach 9) during the course of SHARP. Both projects were eventually cancelled.
Strictly ballistic flight following launch (i.e., without further propulsion after leaving the gun barrel) will not achieve a stable orbit. To achieve a stable earth orbit, additional, guided propulsion is required, typically a secondary propellant, a secondary explosive charge, a rocket motor, or other on-board propulsion and guidance system incorporated into the projectile. One example is disclosed in the paper of Gilreath et al (“The Feasibility of Launching Small Satellites with a Light Gas Gun”; 12th AIAA/USU Conference on Small Satellites, Paper No. SSC98-III-6; 1998). Typically the on-board propulsion (e.g., a rocket motor) would not fire until after an initial period of strictly ballistic flight after launch from the gun barrel. At an appropriate point in the ballistic flight, the on-board propulsion can be fired to achieve orbital insertion. In contrast, if only sub-orbital flight is needed or desired, on-board propulsion and guidance of the projectile may not be needed; strictly ballistic flight could be sufficient in some instances. Likewise, if escape velocity is needed or desired without orbital insertion, then on-board propulsion and guidance may not be required if the muzzle velocity of the gun launch is sufficiently large (i.e., if the muzzle velocity exceeds escape velocity by a margin sufficient to allow for aerodynamic drag on the projectile during its flight). If the muzzle velocity is not large enough, on-board propulsion can be employed to achieve escape velocity.
The initial, ballistic portion of the projectile's flight, after being launched from the gun, is substantially determined by elevation and azimuth of the gun barrel, the muzzle velocity, aerodynamic drag on the projectile, and wind conditions. Typically, the projectile follows a generally parabolic, hyperbolic, or elliptical trajectory. Without additional, onboard propulsion, the projectile either reaches an apogee and falls back to earth, or escapes earth's gravitation altogether (if the muzzle velocity, reduced by aerodynamic drag, exceeds escape velocity). If orbital insertion is desired, or if muzzle velocity alone is insufficient to escape earth's gravity, then additional, on-board propulsion typically is required and can be implemented in a variety of ways.
U.S. Pat. No. 9,273,943 (issued Mar. 1, 2016 to Poulsen on application Ser. No. 14/190,607; both patent and application are incorporated by reference as if fully set forth herein) discloses apparatus and methods for gun launch of a payload into space (sub-orbital, orbital, or exceeding escape velocity) using a projectile with an aerodynamic housing.